The present invention relates to thermal sensor circuits. In particular, the invention relates to thermal sensor circuits for sensing temperature-related characteristics of a semiconductor device.
Integrated circuits (ICs) are generally manufactured on semiconductor substrates (also called wafers) by a process involving deposition. Other semiconductor materials are thermally driven into the substrate. Because of the small size of the ICs, numerous ICs are fabricated using a single dye on the same wafer. The ICs are then separated by cutting. Due to unpredictable variations in the manufacturing process from dye to dye, as well as from wafer to wafer, the characteristics of the individual ICs are not identical. By measuring the characteristics of the manufactured ICs, these variations can be found.
In order to sense the temperature of the IC, a thermal sensor circuit is formed on the chip carrying the IC. If the variations in the temperature sensing characteristics of the sensor circuit are not within an acceptable range, the IC must be discarded as being defective, resulting in a lower IC manufacturing yield. It is therefore desirable to provide compensation for the manufacturing process variations so that the ICs need not be discarded.
The circuit components of the thermal sensor circuit on each IC chip will generally be sufficiently proximate to each other that they will all be affected by the process variations to a similar extent.
An example of a conventional temperature sensor circuit is shown in FIG. 1. The temperature is sensed by comparing the linearly varying voltage at Vsense with the (ideally) fixed reference voltage Vref. For example, if Vsense is 0 volts at 20 degrees Celsius and 1 volt at 120 degrees, for every 10 degrees Vsense increases by 0.1 volts. If it is desired to detect when the temperature reaches 100degrees, Vref should be set to 0.8 volts. When Vsense is less than 0.8 volts, the temperature will be below 100 degrees and the comparator output will be low. When Vsense is greater than 0.8 volts, the temperature will be greater 100 degrees and the comparator output will be high. In order to accurately sense whether the temperature of the IC has passed a particular threshold temperature, Vref must not vary with temperature. Otherwise this will give a spurious result as to the sensed voltage at the output of the comparator. However, due to IC manufacturing process variations, Vref will sometimes vary with temperature.
In order for all the manufactured ICs to meet the required parameters, the process variations need to be taken into account during the design stage of the ICs. The circuits sensitivity to these process parameters must be minimized to get minimal difference in the performance of the circuit from IC to IC.
Transistors Q1 and Q2 are used to generate a voltage across the resistor R1 which is independent of any process variation. This voltage is effectively a property of the Silicon of the transistors and is therefore accurately reproducible. The voltage across the resistor R1 is given by the difference of the base-emitter potentials of transistors Q1 and Q2,
Vbe1xe2x88x92Vbe2=kxc2x7Txc2x7ln(M)
where k is Boltzmann""s constant, T is absolute temperature and M is the ratio (M:1, for M greater than 1) of the emitter areas of Q2 to Q1. The temperature sense voltage Vsense is measured over the temperature sensing resistor R3 and is given by,       V    sense    +            R3      R1        ·    k    ·    T    ·          ln      ⁡              (        M        )            
It is clear from the equation above that the Vsense is not affected by variations in the process. since the process dependent components in the equation, resistors R3 and R1, appear as a ratio and will generally be affected by the process variations to the same extent. The xe2x80x9ccurrent mirror 1xe2x80x9d circuit shown in FIG. 1 is modelled as an ideal p-n-p current mirror for simplicity of explanation.
A reference xe2x80x9cbandgapxe2x80x9d voltage is obtained at the base of Q1 and Q2, such that the value is almost constant over temperature and is given by,       V    ref    =            V      bel        +                  R2        R1            ·      k      ·      T      ·              ln        ⁡                  (          M          )                    
As the temperature varying term of the above equation is small relative to the base-emitter voltage of transistor Q1, Vref changes as Vbe1 changes due to process variations.
FIG. 3a illustrates the relationships of Vsense and Vref over varying voltage and temperature. As can be seen from the plot of FIG. 3a, the normal level of Vref will be crossed by the linearly varying Vsense measurement at the desired temperature level, T0. When the process variations have resulted in changed characteristics of the sensing circuit, this will have the effect of changing the level of Vref so that, for characteristics corresponding to the xe2x80x98process minimumxe2x80x99, Vref will be higher and will be crossed by Vsense at a higher temperature, T2, and for characteristics corresponding to the xe2x80x98process maximumxe2x80x99, Vref will be lower and will be crossed by Vsense at a lower temperature, T1. Temperatures T1 and T2 are spurious results, which, if the temperature differential between these two values is large, can cause an unacceptably high number of occurrences of spurious high temperature alert signals for the IC.
FIG. 3b shows the output of the comparator corresponding to the spurious temperature detections at temperatures T1 and T2 as shown in FIG. 3a. 
It is therefore desirable to reduce the temperature difference (ie. T2xe2x88x92T1) over which spurious detections occur for thermal sensing circuits in order to reduce the number of occurrences of spurious high temperature alert signals for the IC.
The present invention provides a thermal sensor circuit for sensing the temperature of an integrated circuit chip, the thermal sensor circuit including:
an output comparator for comparing a reference voltage, Vref, with a sensed voltage, Vsense, the sensed voltage being measured from a sensing device;
a first circuit to which a reference voltage line is connected to measure Vref;
a first current mirror providing a first current input to the first circuit and to a compensation circuit;
a second current mirror providing a second current input to the compensation circuit and to the sensing device; and wherein
the compensation circuit provides a current gain, defined as the ratio of the second current input to the first current input, for compensating for variations in Vref due to variations of the characteristics of the thermal sensing circuit arising from manufacture by adjusting the second current input in dependence on the variations of the characteristics to thereby vary Vsense with Vref.
Preferably, the compensation circuit includes first, second, third and fourth bipolar junction transistors (BJTs) wherein:
the first BJT has a collector terminal connected to the first current input of the first current mirror, a base terminal connected to a common base connection and an emitter terminal connected to ground;
the second BJT has a collector terminal connected to the second current input of the second current mirror, a base terminal connected to the common base connection and an emitter terminal connected to ground;
the third BJT has a collector terminal connected to the second current input, a base terminal connected the first current input and an emitter connected to the common base connection;
the fourth BJT has a collector terminal connected to a voltage supply of the thermal sensor circuit, a base terminal connected to the common base connection and an emitter terminal connected to ground; and
the ratio of emitter area of the fourth BJT to the emitter areas of the first, second and third BJTs is N:1, where N greater than 0.
Preferably, the first circuit includes fifth and sixth BJTs, wherein:
the fifth BJT has a collector terminal connected to the first current input, a base terminal connected to the reference voltage line and an emitter terminal connected to an output point of the first circuit via a first resistor;
the sixth BJT has a collector terminal connected to the first current input, a base terminal connected to the reference voltage line and an emitter connected to the output point of the first circuit;
the output point of the first circuit is connected to ground via a second resistor.
Preferably, the ratio of emitter area of the fifth BJT to the emitter area of the sixth BJT is M:1, where M greater than 1. Preferably, each of the first to sixth BJTs is an n-p-n transistor.
Preferably, the current gain is given by:       I2    I1    =                    β        2            +                        (                      3            +            N                    )                ⁢        β                            β        2            +      β      +              (                  2          +          N                )            
where:
I1 is the first current input;
I2 is the second current input; and
xcex2 is the common-emitter current gain of each of the first to sixth BJTs.
Preferably, the first and second current mirrors are connected to the voltage supply of the thermal sensor circuit and use p-n-p BJTs to supply the first and second current inputs, respectively.
Advantageously, the thermal sensor circuit provides a compensation function which reduces the temperature range over which spurious temperature detection signals are sent by the comparator by providing a compensation circuit which provides current gain to adjust Vsense according to the degree of process variations effected by the manufacturing process of the IC on the thermal sensing circuit.